is there anything that you personally ARE aware of?
Since most of what people “know” is really just conjecture and hearsay (unexplained trips out of town, sudden weight loss, police cars making house calls) there is really no way to KNOW if there are illnesses, or sholom bayis problems. So being evasive is not really an outright lie.
Thats why its crucial to talk to the perspective person’s Rov / Rebbe. They KNOW, because they’ve been told.
That said, if I cannot give a 100% stellar, squeaky clean report about someone, I tell the person, I’m on my way out to maariv (or wherever), let me call you back in < x time> minutes.
Then I call the person, and tell them, so and so is asking about you. How do I explain <whatever the issue>.
But B”H everyone I’ve been called (or place a call) about is someone I would consider for a match myself, so I can be as truthful as possible.